Pneumatic conveying systems are extensively used in industrial plants to move bulk material from point to point through tubes. In a pneumatic conveying system, the bulk material is in the form of particulate solids in a stream of air or other gas. The gas conveys the particulate bulk material through a conduit, usually in the form of a tube.
Pneumatic conveying systems operate at different gas velocities depending upon the particulate material being conveyed or the mode of conveying. The highest gas velocities are employed in the dilute phase conveying mode wherein the particulate is carried by a gas velocity typically between 3200 and 6000 feet per minute and is caused to stream through the conveying tube at high speed and with minimum contact against the tube walls.
The lowest gas velocities are employed in the dense phase conveying mode wherein the particulate moves through the conveying tube at speeds as low as 7 to 10 feet per second, in the form of a piston, or series of discrete pistons, which are in substantial contact with the tube walls.
In a typical industrial application of a pneumatic conveying system, it is desired to convey bulk material from one of a plurality of sources to at least one of a plurality of destinations or "terminals". In order to accomplish routing of the flow path as needed for various applications, a number of schemes have been utilized. A simple type of system is a manual hose switch station. In a hose switch station, a number of input tubes terminate in flexible hoses, which are manually connected as needed to a plurality of possible output tubes. The manual hose switch station has several disadvantages. It requires a person to be in physical proximity to the tubes, which may be hazardous if the particulates are poisonous or corrosive or if there is an accidental release of high pressure gas and particulates. Because the hoses must be physically handled, and the operation is awkward and arduous, it is generally accepted that the maximum diameter of these hoses be limited to six inches. There is leakage of gas out of hoses and couplings in a manual hose switch station, which has been known to waste as much as 5% of the installed gas blower horsepower. The system must be shut down for a considerable period whenever the tubes are switched. Further, it is generally preferred in industrial applications that pneumatic conveyor systems be operated from a central station, which precludes manual hose switching stations and mandates the use of remotely-operable diverter valves.
Industry presently prefers the use of a two-way diverter valve as the basic remote control switching device. A single valve permits the flow material from a source tube to be directed to either of two outlet tubes exiting the valve.
For situations where it is desired to direct flow from a source tube to three or more exit tubes, there are diverter valves with more than two outlets, but these valves have been found to be extremely complicated, expensive, and unreliable. In situations requiring three or more outlet tubes, it is customary to cascade a number of two-way valves; for example, for a three-outlet system, the source tube will first branch into two outlet tubes, and then one of the outlet tubes will then branch into two further outlet tubes, thus resulting in ultimately three outlet tubes. The number of two-way valves required in these situations is the number of output tubes (or terminals), minus one. Obviously, a requirement for a large number of terminals will make for a large number of two-way diverter valves, and complicate matters of selecting the correct valves to open.
In like manner, when it is required to pneumatically transport material from one of a plurality of sources into a single terminal, it is customarily done with two-way diverter valves.
There are some industrial situations where there are a plurality of sources of material to be conveyed, and a plurality of terminals to which the material is to be directed, and any one of the sources may need to be connected to any one of the terminals at a given time. In order to accomplish this with two-way diverter valves, trees of diverter valves must be associated with sources and terminals. In the production of plastics, for example, a typical large plant might have four pneumatic conveying systems coming from four rows of quality control silos and it may be necessary to direct the flow of any of the four systems to any of the twelve storage silos. Further, it may be necessary that all four systems be able to operate simultaneously. This arrangement could require 48 two-way diverter valves, each, typically, having an air operator, an electro-pneumatic valve, and two position limit switches. The arrangement of conveying tubes would be very complicated and require a great deal of space, compressed air would have to be piped to the 48 valves, and the electric wiring would be extensive, leading to a great capital cost and high maintenance in operation As a result, plastics plants often reject this option and install hose switching stations, with their attendant disadvantages as mentioned above.
It is an object of the present invention to provide a device for changing flow paths by remote control in pneumatic conveying systems, particularly in systems having at least one source to be conveyed in a pneumatic stream and a plurality of terminals.